1. Field of the Invention
The present invention relates to fluorescent lamps and more particularly to an electronic ballast for use with such fluorescent lamps.
2. Background Art
Fluorescent lamps usually require a ballast circuit for producing an alternate current signal with a high voltage amplitude for effective operation of the fluorescent lamp. Ballasts of this type are typically separated into two broad categories, the first being of the electromagnetic type while the second is of a true electronic form.
The ballast is a device which performs the following functions:
(a) apply a high voltage across the lamp, in order to fire an arc in the lamp, and
(b) limit the current through the lamp, once the arc has been fired.
In the electronic ballast, a resonant element is used to provide the initial starting voltage to the lamp and also to limit the current through the lamp, once the arc has been struck.
A background art search directed to the subject matter of this application and conducted in the U.S. Patent and Trademark Office disclosed the following U.S. Pat. Nos. 4,175,246, 4,472,661, 4,553,070, 4,722,040, 5,034,660, 5,063,331.
None of the patents uncovered in the search disclosed means converting current into voltage or an included transformer therein, where depending upon direction of current, either positive or negative voltage applied across the transformer and hence associated switching elements where switch over occurs at zero current crossing.
In most self-commutating series resonating circuits, such as found in the prior art, the transformer included serves as the commutating element. It has been discovered that the transformer typically has two modes of operation. In such an arrangement, the transformer senses the zero-crossing of its primary current and hence causes the polarity of the secondary voltages to change. This then causes the associated switching transistors to switch over when current through them is zero, thus minimizing stress on the transistors.
The transformer also senses a decrease in its primary current and hence causes the polarity of the secondary voltages to change. This causes the transistor to switch over when the current through them is at maximum value. This operation then greatly increases the losses in the transistor and also increases stress on them.
While operation in the first mode is considered desirable, the actual mode of operation of the transformer is a function of the primary current depending thus on the construction of the transformer, etc. In fact, both modes of operation have been observed in the same circuit at different intervals of time.